super reverb problem

I'm wondering if someone can help me with my vintage 65 super reverb. First it has been serviced and has all problem caps changed,new jj tubes installed.It sounds fabulous but after playing for an hour or so it starts to distort especially when you pluck the lower strings,it will cut in and out. The sound is sort of like an extreme distortion pedal. I've had it on the bench and tried to create the problem with a chop stick,but all seems good, anybody got any ideas as to what might be breaking down? This is my favorite amp!! Thanks for any help

Sounds like you might be experiencing some transitory power supply sag, possibly thermal-related. You might try swapping out your rectifier tube with a new GZ34 an' see what that does. Could also be (an' I'm hesitant to say it) your power transformer may be headin' south. Not beyond the realm of possibility, 'specially if'n the PT is original to the amp.

Best of luck, HTH

When this starts tojust happen turn the lights in the room down to see if the plates of the output tubes are starting to glow red or even a very small round spot of purple, if so you have a bias problem that will weaken new output tubes quite fast.

Thanks for your help, I did notice when I went to bias the power tubed that the readings were very different, 35ma on one tube and 27ma on the other, the voltage was 470 on both tubes,could this be a possible cause of the problem?